I would take a serious look at the first reply about $$. I just (tonight!!) finished my room and put a couple girls in there. I have yet to buy a tray, more pots, grow medium, nutes, and other random necessities and my receipts total $900! It is only a 4x4x9 room and a good pal loaned me the light!! It is an expensive startup, so think about how much you want to spend.
If It were my 6x6 I would throw 2 600's in there, veg fewer plants more and create some monsters that will have good yields. Also, since it is your first grow, less plants may be easier to learn with.
Another consideration is that though it is expensive, Don't Skimp! Get good equipment and READ!! This website is a goldmine!
As far as yield...I am shooting for 1 pound with my first run in a new 4x4 room with 5 plants. If it were my first grow I would try not to think about it, just about dialing in the environment so the plants can be as healthy as possible.
Good luck!